Process Accumulated 8% Efficient Cu2ZnSnS4‐BiVO4 Tandem Cell for Solar Hydrogen Evolution with the Dynamic Balance of Solar Energy Storage and Conversion

Abstract A process accumulated record solar to hydrogen (STH) conversion efficiency of 8% is achieved on the Cu2ZnSnS4‐BiVO4 tandem cell by the synergistic coupling effect of solar thermal and photoelectrochemical (PEC) water splitting with the dynamic balance of solar energy storage and conversion of the greenhouse system. This is the first report of a Cu2ZnSnS4‐BiVO4 tandem cell with a high unbiased STH efficiency of over 8% for solar water splitting due to the greenhouse device system. The greenhouse acts as a solar thermal energy storage cell, which absorbs infrared solar light and storage as thermal energy with the solar light illumination time, while thermoelectric device (TD) converts thermal energy into electric power, electric power is also recycled and added onto Cu2ZnSnS4‐BiVO4 tandem cell for enhanced overall water splitting. Finally, the solar water splitting properties of the TD‐Cu2ZnSnS4‐BiVO4 integrated tandem cell in pure natural seawater are demonstrated, and a champion STH efficiency of 2.46% is presented, while a large area (25 cm2) TD‐Cu2ZnSnS4‐BiVO4 integrated tandem device with superior long‐term stability is investigated for 1 week, which provides new insight into photoelectrochemical solar water splitting devices.

Infrared light accounts for approximately 50% of solar radiation energy.
However, the infrared light is normally absorbed in the form of thermal energy rather than photons. As shown in Fig. S1, in approximately 160 minutes, the temperature of the buffer solution rose from 293 K to 328 K under solar simulated AM 1.5 G irradiation, and then reached the equilibrium point. This part of the high temperature is caused by the infrared light in the sunlight, but the infrared light cannot be absorbed by the photoelectrode, in our previous study, this part of the heat energy was not fully utilized. In this study, in order to maximize the utilization of the full spectrum of the sunlight, we integrated TD system to convert this part of the thermal energy into electrical energy. We found that with the integration of the TD system, in approximately 130 minutes, the temperature of the buffer solution was finally maintained at about 305 K. The TD system not only produced a significant additional In order to test the effect of electrolyte temperature on water splitting, we tested the PEC performance of the photoelectrode in different temperature ranges. Fig. S2 shows that the current density of CZTS-based photocathode and BiVO 4 photoanode increased with increasing temperature. It was found that the PEC performance of CZTS-based photocathode and BiVO 4 photoanode slightly increased with temperature.
The increment in temperature enhanced the activity of the catalyst and improved the charge transfer speed at the photoelectrode interface. [1] Similar results were already reported. [2][3] However, by comparing with Fig. 6 and 7, we can think that the improved PEC performance caused by temperature change is negligible compared with the improvement of photoelectrode performance after integrating thermoelectric device.  peaks on the surface of HfO 2 / CdS/CZTS ( Fig. S4b and c), but no Cd peak can be observed (Fig. S4d), indicating the HfO 2 layer entirely covered the surface of CdS/CZTS film. As shown in Fig. S5a, we can clearly see that the deposited CdS and HfO 2 buffer layers can effectively improve the incident photon to current efficiency (IPCE) of CZTS photocathode, and the HfO 2 /CdS/CZTS photoelectrode has higher incident photon to current efficiency than bare CZTS. This is because the CdS layer can form a PN junction with the CZTS absorption layer, which can effectively improve the separation efficiency of photogenerated charges in the semiconductor. At the same time, the HfO 2 protective layer deposited on the CZTS can effectively passivate the interface and inhibit the photoelectrode surface photocorrosion, which further reduce the resistance and improve photon absorption efficiency (Fig. S5b). The trend in EIS curves also similar with their PEC performances as shown in Fig. 4.  CdS and HfO 2 were reference to previous publications. [4][5]   and Rct in the equivalent circuit diagram represent series resistance and interface charge transfer resistance, respectively. [6] The fitted values of each components are shown in Table S1. The Rs of BiVO 4 and TD-BiVO 4 are similar, indicating that the effect of series resistance is negligible. [7] After integrating the TD, the Rct of TD-BiVO 4 is significantly reduced, indicating that the voltage provided by the TD effectively promotes the charge separation and transfer efficiency, thereby enhancing the photocurrent density of BiVO 4 . As shown in Fig. S9b, the M-S plots of BiVO 4 and TD-BiVO 4 exhibited the positive expected slope, indicating that the applied thermoelectric voltage did not change the hole conductivity type of BiVO 4 , and the flat-band potential of BiVO 4 negatively shifts from 0.21 V to 0.08 V, which is due to the presence of a large number of surface states on the BiVO 4 surface and the reduction of the surface Fermi level pinning effect caused by the applied voltage. [8] Finally, we tested the Open circuit potential (OCP) of BiVO 4 and TD-BiVO 4 . As shown in Fig. S9c, with the aid of the TD, the OCP of BiVO 4 was increased from 0.3 V to 1.6 V, the high OCP is beneficial to improve the efficiency of water splitting, indicating that the thermoelectric device further optimized the PEC performance of  While the dark current is close to zero when light is turned off. Fig. S10b shows the photocurrent density of the TD-BiVO 4 photoanode is 13 mA/cm 2 (1.23 V RHE ). Fig.   S10c shown the photocurrent density of the CZTS-BiVO 4 tandem cell under the TDdrive can reach about 6.6 mA/cm 2 during the hydrogen production process, which is a very significant improvement compared to the cell without TD-drive, which is only about 1 mA/cm 2 . It is worth noting that the photocurrent of the CZTS-BiVO 4 tandem cell is 0 mA/cm 2 regardless of whether there is TD or not. This is because the voltage provided by the TD (about 1 eV) is much smaller than the band gap of CZTS (1.45 eV) and BiVO 4 (2.3~2.4 eV). In the absence of sunlight, the electrons in the semiconductor cannot obtain enough energy through photoexcitation and transition to the conduction band, so conduction cannot be formed, and the entire TD-CZTS-BiVO 4 tandem cell circuit is in a disconnected state.

Table S1
Impedance fit values for each component of BiVO 4 and TD-BiVO 4 .

Table S2.
Chemical reaction formula and Normal Hydrogen Electrode (NHE) under different conditions.

Table S3
Comparison of main ions in natural seawater and buffer solution (ion concentration exceeds 100 mg/L).